Elementary steps of the catalytic NOx reduction with NH3: Cluster studies on reaction paths and energetics at vanadium oxide substrate

Gruber, Mathis; Hermann, Klaus

doi:10.1063/1.4849556

Cited by 17 publications

(24 citation statements)

References 41 publications

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“…The substitution of a metal cation from a metal oxide host with a dopant atom can have a significant impact on the properties of the total system and therefore change its behavior either in a constructive or destructive manner for the given application. At last to investigate the effect of the modified Brønsted acidity on the SCR activity we can calculate E act = DE BD and E ts = DE AD with a substitute and evaluate how it affects the rate at a given temperature according to eqn (11)- (13). [44][45][46][47] In order to investigate the effect we substituted a single Ti atom from the support with 8 different metal ions: Si, V, Ge, Se, Zr, Sn, Te, and Hf, expected to act as isovalent dopants.…”

Section: The Influence Of a Dopant Substitutionmentioning

confidence: 99%

“…sulfur deactivation it will continue to be a major catalysts in the SCR industry. 2,[5][6][7][8][9][10][11][12][13] It is generally believed that the reaction occurs through an Eley-Rideal type mechanism where the first step is ammonia adsorption on the catalytic surface followed by an adsorption and reaction with either a weakly adsorbed or gas phase NO to form an activated complex that decomposes into N 2 and H 2 O leaving behind a reduced vanadium site. 4 To accomplish this it is important to elucidate the key fundamental aspects of the reaction mechanisms and the role of the active sites of the catalyst e.g.…”

Section: Introductionmentioning

confidence: 99%

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The reaction mechanism for the SCR process on monomer V⁵⁺sites and the effect of modified Brønsted acidity

Arnarson

Falsig

Rasmussen

et al. 2016

Phys. Chem. Chem. Phys.

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The energetics, structures and activity of a monomeric VO3H/TiO2(001) catalyst are investigated for the selective catalytic reduction (SCR) reaction by the use of density functional theory (DFT). Furthermore we study the influences of a dopant substitute in the TiO2 support and its effects on the known properties of the SCR system such as Brønsted acidity and reducibility of vanadium. We find for the reduction part of the SCR mechanism that it involves two Ti-O-V oxygen sites. One is a hydroxyl possessing Brønsted acidity which contributes to the formation of NH4(+), while the other accepts a proton which charge stabilizes the reduced active site. In the reduction the proton is donated to the latter due to a reaction between NH3 and NO that forms a H2NNO molecule which decomposes into N2(g) and H2O(g). A dopant substitution of 10 different dopants: Si, Ge, Se, Zr, Sn, Te, Hf, V, Mo and W at each of the sites, which participate in the reaction, modifies the energetics and therefore the SCR activity. We find that Brønsted acidity is a descriptor for the SCR activity at low temperatures. Based on this descriptor we find that Zr, Hf and Sn have a positive effect as they decrease the activation energy for the SCR reaction.

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Section: The Influence Of a Dopant Substitutionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The reaction mechanism for the SCR process on monomer V⁵⁺sites and the effect of modified Brønsted acidity

Arnarson

Falsig

Rasmussen

et al. 2016

Phys. Chem. Chem. Phys.

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“…[2] Thea dvent of powerful computational resources,t he search for catalyst formulations counteracting the implicit drawbacks of vanadium-based catalysts, [3] and the development of low-temperature SCR catalysts for automotive applications, have rejuvenated the debate on the nature of the active center. [6] By comparison, L NH 3 is more resistant to desorption at agiven temperature [7] and demonstrates higher heat of adsorption than B NH 3 . TheB rønsted and Lewis acid sites are responsible for the adsorption of NH 3 and have thus been anticipated to be essential for the reaction mechanism.…”

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confidence: 99%

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium‐Based Catalysts

et al. 2016

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The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH 3 )o nv anadium-based catalysts rely on the involvement of Brønsted or Lewis acid sites.T his issue has been clearly elucidated using ac ombination of transient perturbations of the catalyst environment with operando time-resolved spectroscopyt oobtain unique molecular level insights.N itric oxide reacts predominantly with NH 3 coordinated to Lewis sites on vanadia on tungsta-titania (V 2 O 5 -WO 3 -TiO 2 ), while Brønsted sites are not involved in the catalytic cycle.The Lewis site is amono-oxovanadyl group that reduces only in the presence of both nitric oxide and NH 3 .Wewere also able to verify the formation of the nitrosamide (NH 2 NO) intermediate,which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach,d emonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.Abatement of harmful NO x is amajor objective of environmental and health protection policies as ar esult of the increasing emissions from power plants and vehicles in rapidly developing countries.E fficient NO x removal from exhaust gases of stationary sources is achieved by reacting NH 3 on as olid catalyst, typically consisting of dispersed vanadia on tungsta-titania (V 2 O 5 -WO 3 -TiO 2 ), according to the standard selective catalytic reduction (SCR) process. [1] 4NOþThei ncreasingly stringent emission regulations in the transport sector also make this technology attractive for controlling the exhaust of mobile sources;e specially diesel trucks,r ailroad diesel engines,a nd more recently,s hip engines.Despite the widespread use of SCR, some molecular aspects of the reaction mechanism remain controversial. [2] Thea dvent of powerful computational resources,t he search for catalyst formulations counteracting the implicit drawbacks of vanadium-based catalysts, [3] and the development of low-temperature SCR catalysts for automotive applications, have rejuvenated the debate on the nature of the active center. Themechanistic aspects of SCR using vanadium were summarized by Busca et al. [4] Ther eaction proceeds according to the stoichiometry of Reaction (1) and the two nitrogen atoms of the N 2 product originate,o ne each, from NO and NH 3 .I ti sg enerally accepted that the mechanism comprises an acid site where NH 3 is activated and ar edox site that requires oxygen to be regenerated. TheB rønsted and Lewis acid sites are responsible for the adsorption of NH 3 and have thus been anticipated to be essential for the reaction mechanism. While Brønsted acid sites bind NH 3 to form NH 4 + (B NH 3 ), Lewis acid sites coordinate NH 3 directly with ametal atom (L NH 3 ). NO reacts on the acid sites according to an Eley-Rideal mechanism, accompanied by the reduction of V 5+ to V 4+ .I ti sa lso recognized that the role of oxygen is to restore the V 5+ O x active center. Va rious experimental and theoretical studies hav...

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“…Extensive research has been undertaken to understand the different aspects of standard V-based SCR catalysts. These efforts were aimed at revealing mechanistic details such as the nature of the active centers and the rate determining steps [10][11][12][13][14][15][16][17], the origin of catalyst aging [18] and the electronic interactions between the various catalyst components and the poisoning phenomena [19][20][21]. For this purpose, parameters such as the synthesis of the TiO2 support material [22,23], the synthesis method, the loading and the nature of WO3 and V2O5 and the structural change upon thermal/hydrothermal treatment have been addressed.…”

Section: Introductionmentioning

confidence: 99%

VOx Surface Coverage Optimization of V2O5/WO3-TiO2 SCR Catalysts by Variation of the V Loading and by Aging

et al. 2015

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V2O5/WO3-TiO2 selective catalytic reduction (SCR) catalysts with a V2O5 loading of 1.7, 2.0, 2.3, 2.6, 2.9, 3.2 and 3.5 wt. % were investigated in the fresh state and after hydrothermal aging at 600 °C for 16 h. The catalysts were characterized by means of nitrogen physisorption, X-ray diffraction and X-ray absorption spectroscopy. In the fresh state, the SCR activity increased with increasing V loading. Upon aging, the catalysts with up to 2.3 wt. % V2O5 exhibited higher NOx reduction activity than in the fresh state, while the catalysts with more than 2.6 wt. % V2O5 showed increasing deactivation tendencies. The observed activation and deactivation were correlated with the change of the VOx and WOx surface coverages. Only catalysts with a VOx coverage below 50% in the aged state did not show deactivation tendencies. With respect to tungsten, above one monolayer of WOx, WO3 particles were formed leading to loss of surface acidity, sintering, catalyst deactivation and early NH3 slip. An optimal compromise between activity and hydrothermal aging resistance could be obtained only with V2O5 between 2.0 and 2.6 wt. %.

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Elementary steps of the catalytic NOx reduction with NH3: Cluster studies on reaction paths and energetics at vanadium oxide substrate

Abstract: We consider different reaction scenarios of the selective catalytic reduction (SCR)

Cited by 17 publications

References 41 publications

The reaction mechanism for the SCR process on monomer V⁵⁺sites and the effect of modified Brønsted acidity

The reaction mechanism for the SCR process on monomer V⁵⁺sites and the effect of modified Brønsted acidity

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium‐Based Catalysts

VOx Surface Coverage Optimization of V2O5/WO3-TiO2 SCR Catalysts by Variation of the V Loading and by Aging

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Elementary steps of the catalytic NOx reduction with NH3: Cluster studies on reaction paths and energetics at vanadium oxide substrate

Abstract: We consider different reaction scenarios of the selective catalytic reduction (SCR)

Cited by 17 publications

References 41 publications

The reaction mechanism for the SCR process on monomer V5+sites and the effect of modified Brønsted acidity

The reaction mechanism for the SCR process on monomer V5+sites and the effect of modified Brønsted acidity

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium‐Based Catalysts

VOx Surface Coverage Optimization of V2O5/WO3-TiO2 SCR Catalysts by Variation of the V Loading and by Aging

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The reaction mechanism for the SCR process on monomer V⁵⁺sites and the effect of modified Brønsted acidity

The reaction mechanism for the SCR process on monomer V⁵⁺sites and the effect of modified Brønsted acidity